The present invention relates to television systems in general and more particularly to a digital circuit for generating the vertical synchronizing signal and identification signals for the first and second fields in digital television receivers.
Digital television receivers are now employed in the processing of the analog television signal. Such digital television receivers typically employ an analog to digital converter whereby the analog television signal is converted into a digital signal. The digital signal may be stored in a memory and further processed in order to obtain various different displays as well as many additional features obtained by operating on the stored digital signal. In this manner the digital signal is then converted by means of a digital to analog converter back to an analog signal where it is applied to a typical television receiver picture tube. The basis of any video digital technique involves quantizing the analog video signal indicative of a television picture into a number of discrete brightness levels or pixels. The video signal is sampled at a rate normally greater than the Nyquist rate. In practice some 256 levels may be used corresponding to an 8 bit data word.
As one will understand, modern television standard employ interlaced scanning techniques. Interlace scanning is achieved by making the horizontal (line scanning) rate an odd multiple of one half the vertical (field scanning) rate. In the United States standards the horizontal rate is 15,750 lines per second. In other words at 30 frames per second the scanning pattern has 525 lines per frame and 262.5 lines per field. In color transmission, the horizontal rate is 15,734.264 lines per second and the frame rate is 29.97 per second. An equivalent statement is that interlacing is achieved when the number of lines per frame is an odd number, thus requiring each field to have an even number of lines plus one half line. Essentially, the typical NTSC system employs two fields in one frame to accomplish a complete television picture scan. Throughout the word there are several different scanning standards in use but all have in common interlaced scanning. In Europe one employs 625/50 standard as for
which uses 625 lines per television picture with each picture formed by two interlaced fields at a repetition rate of 50 Hz. The NTSC system, as described above, uses 525/60 standard.
As one can understand, with the advent of digital television receivers, one implements many of the processing techniques by means of digital circuits. Digital circuits, as one will ascertain, are easier to implement and provide more reliable operation than typical analog circuits. Accordingly, digital circuits can be easily employed and fabricated by use of integrated circuits. The use of the integrated circuit enables many of the modular components of a digital television receiver to be implemented.
As indicated above, in order to properly operate a television receiver one must recover the vertical synchronizing signal and further one must obtain identification signals for the first and second fields in a television frame. These signals of course are important to the digital television receiver operation.
As will be explained, according to the present invention, a horizontal oscillator, produces horizontal deflection signals and an internal horizontal synchronizing signal with a 1:1 mark/space ratio the oscillator is operated in a phase lock loop. The use of the phase lock loop eliminates the necessity of performing the internal generation of the vertical synchronizing signal and the generation of field identification signals by means of subcircuits which are separated and separately implemented from the horizontal oscillator.
The format to be described results in a reduction of circuit complexity which in turn leads to an improved utilization of an integrated circuit chip area, if the circuit is implemented as a semiconductor monolithic integrated circuit. The invention to be described also offers the advantages inherent in conventional internal vertical synchronizing signal generation.
It is therefore an object of the present invention to provide an improved digital circuit for generating the vertical synchronizing signal and identification signals for the first and second fields to be employed in a digital television receiver.
It is a further object to provide such a digital circuit which can be easily implemented by means of integrated circuit techniques.